The present invention relates to an information recording medium which is composed of a substrate, underlayer, and recording film and is capable of recording information by means of depressions or pits which are formed on the recording film upon irradiation with a recording energy beam.
More particularly, the present invention relates to an information recording medium which is capable of recording digital information in a real-time mode by irradiation with a recording energy beam such as laser light and electron beam, said digital information including video and audio signals (formed by FM modulation of analog signals), computer-generated data, facsimile signals, and digital audio signals.
The information recording medium capable of recording information by means of depressions or pits which are formed on the recording film upon irradiation with a recording energy beam, is composed of a substrate, underlayer, and recording film. The substrate is a glass disc or a plastics disc (of acrylic resin or the like) provided with a UV light-cured resin layer in which are formed tracking grooves, or a plastics disc (of polycarbonate, acrylic resin, polyolefin, or the like) having tracking grooves formed by injection molding. The underlayer is made from a solvent-soluble resin (such as nitrocellulose, acetyl cellulose, and copper phthalocyanine) or a sputterable resin (such as fluoroplastics). The recording film is a thin film composed mainly of Te.
It has been a common practice to stabilize the recording film of the information recording medium by providing, on at least one side of the recording film, with a thin film as a supporting layer for recording, which is composed mainly of an element such as Sb, Si, Ge, and Sn. (See Japanese Patent Laid-open No. 224446/1983.)
The above-mentioned recording medium based on the prior art technology has a disadvantage that the depressions or pits (simply referred to as pits hereinafter) which are formed upon irradiation with an energy beam have a small rim width and a large rim inner diameter. (This disadvantage is due to the lack of full attention to the temperature distribution in the part irradiated with an energy beam, the viscosity of the molten recording film, and the wettability of the underlayer by the molten recording film.) Since the signals reproduced by recorded pits are dependent on the rim inner diameter of pits, pits with a smaller rim inner diameter are more suitable for high-density recording than pits with a larger rim inner diameter, even though they are the same in rim outer diameter. This presents a problem associated with the low power margin of recording light (the power region of the recording light which secures the sufficient reproducing signal intensity) when high-density recording is to be made on the conventional recording medium.